1. Field of the Invention
The present invention relates to a dual mode band-pass filter used, for example, as a band filter in a communication apparatus operating in a band that ranges from the microwave band to the millimeter wave band. In particular, the present invention relates to a dual mode band-pass filter having an improved dielectric body.
2. Description of the Related Art
As band-pass filters used in the high-frequency range, various types of dual mode band-pass filters have been ARE known (e.g., refer to MINIATURE DUAL MODE MICROSTRIP FILTERS, J. A. Curtis and S. J. Fiedziuszko, 1991 IEEE MTT-S Digest).
FIGS. 5 and 6 are schematic diagrams showing conventional dual mode band-pass filters.
In a band-pass filter 200 shown in FIG. 5, a circular conductive film 201 is provided on a dielectric body (not shown in the drawing). An input-output coupling circuit 202 and an input-output coupling circuit 203 are coupled with the conductive film 201 and are arranged to be perpendicular to each other. An open-top stub 204 is arranged to be at a central angle of 45xc2x0 with respect to the input-output coupling circuit 203. As a result, two resonant modes having different resonant frequencies are combined so that the band-pass filter 200 functions as a dual mode band-pass filter.
In a dual mode band-pass filter 210 shown in FIG. 6, a substantially square conductive film 211 is provided on a dielectric body. Input-output coupling circuits 212 and 213 are coupled with the conductive film 211 and are arranged to be perpendicular to each other. One corner, which is positioned at an angle of 135xc2x0 with respect to the input-output coupling circuit 213, is chamfered. As a result of a chamfered section 211a being provided, two resonant modes have different resonant frequencies, and the two resonant modes are combined so that the band-pass filter 210 functions as a dual mode band-pass filter.
On the other hand, dual mode filters using ring-shaped conductive films instead of circular conductive films are also known, as disclosed in Japanese Unexamined Patent Application Publication Nos. 9-139612 and 9-162610. That is, in such a dual mode filter, a ring transmission line is used, and in a manner similar to that in the dual mode band-pass filter shown in FIG. 5, input-output coupling circuits are arranged to be perpendicular to each other, and also an open-top stub is provided on a portion of the ring transmission line.
Japanese Unexamined Patent Application Publication No. 6-112701 also discloses a dual mode filter using a similar ring transmission line.
In the various types of conventional dual mode band-pass filters described above, as the dielectric body, for example, a BaOxe2x80x94Al2O3xe2x80x94SiO2 type ceramic body containing BaO, Al2O3, and SiO2 as main constituents, or a body composed of a synthetic resin is used.
In the conventional dual mode band-pass filter, by forming one conductive film pattern, a double band-pass filter can be constructed, and thus the band-pass filter can be miniaturized.
However, in the conductive pattern having a particular shape, since input-output coupling circuits must be spaced at a predetermined angle, it is not possible to increase the degree of coupling, and a broader pass band cannot be achieved.
Additionally, since the shape of the conductive film is limited, design versatility is low.
Moreover, in the conventional dual mode band-pass filter, since the frequency band used is up to several GHz at most, the dielectric loss of the dielectric body, i.e., the Q value of the dielectric, need not be taken into account.
In general, the Q value of a dielectric decreases as the frequency decreases. That is, as the frequency increases, the dielectric loss increases. For example, a dielectric body made of the BAS material has a Q value of approximately 300 at 10 GHz, and the dielectric loss is greatly increased at the frequency band of 10 GHz or more.
Therefore, in conventional band-pass filters, the insertion loss is high in the high-frequency range.
In order to overcome the problems described above, preferred embodiments of the present invention provide a dual mode band-pass filter that is miniaturized, has greatly increased design versatility, and a greatly reduced insertion loss at the high-frequency range.
In accordance with a preferred embodiment of the present invention, a dual mode band-pass filter includes a dielectric body having first and second main surfaces, a metal film partially provided on the first main surface of the dielectric body or at a predetermined level in the dielectric body, the metal film being provided with an opening, a protrusion, or a cut-out arranged to combine two resonant modes, at least one ground electrode disposed on the second main surface or in the interior of the dielectric body so as to be opposed to the metal film with a portion of the dielectric body therebetween, and a pair of input-output coupling circuits being coupled with different portions of the metal film. The dielectric body is preferably made of a dielectric ceramic containing a ceramic and a glass as main constituents, is capable of being fired simultaneously with any one of Cu, Ag, and Au, and has a Q value of more than approximately 300 at about 10 GHz.
Consequently, since the shape of the metal film is not particularly limited, it is possible to increase design versatility, and it is also possible to easily provide dual mode band-pass filters having various bandwidths. When the metal film and the ground electrodes are made of Cu, Ag, or Au, the dielectric body can be fired simultaneously with the metal film and the ground electrodes, and it is possible to produce a band-pass filter efficiently using a known integrated ceramic firing technique. Furthermore, since the Q value is more than approximately 300 at about 10 GHz, it is possible to construct a dual mode band-pass filter in which the insertion loss is small.
Preferably, in the dual mode band-pass filter, the dielectric body includes (A) MgOxe2x80x94MgAl2O4-based ceramic powder and (B) glass powder containing about 13% to about 50% by weight SiO2, about 3% to about 60% by weight B2O3, and about 0% to about 20% by weight Al2O3.
When the dielectric ceramic containing the MgOxe2x80x94MgAl2O4-based ceramic powder and the glass powder having the predetermined composition as described above is used, the Q value is approximately 400 or more at about 10 GHz, and thus it is possible to further decrease the insertion loss.
More preferably, the glass powder contains at least one alkaline-earth metal oxide selected from the group consisting of BaO, SrO, CaO, and MgO in an amount of about 10% to about 40% by weight of the total glass powder.
The alkaline-earth metal described above decreases the melting temperature during the formation of glass and also acts as a crystal constituent in crystallized glass. If the content of the alkaline-earth metal oxide is less than about 10% by weight, the melting temperature may be increased. If the content exceeds about 40% by weight, the amount of crystal precipitation increases and the strength of the body may be decreased.
Since it is possible to decrease the melting temperature during the formation of glass, the firing cost of the dielectric body is greatly reduced.
Preferably, the glass powder preferably contains at least one alkali metal oxide selected from the group consisting of Li2O, K2O, and Na2O in an amount of about 10% by weight or less of the total glass powder, and more preferably, in an amount of about 2% to about 5% by weight. The alkali metal oxide decreases the melting temperature during the formation of glass. Consequently, the cost of formulating glass powder can be reduced and also it is possible to prevent the Q value from decreasing. If the content of the alkali metal oxide exceeds approximately 10% by weight, the Q value may be decreased.
Preferably, the dielectric body contains about 15% by weight or less ZnO, and more preferably, about 10% by weight or less. The zinc oxides decrease the firing temperature. Due to the zinc oxides contained, a dense dielectric body can be obtained. If the content of the zinc oxides as ZnO exceeds about 15% by weight, it may not be possible to obtain a dense sintered compact.
Additionally, the zinc oxides may be provided as glass components.
Preferably, the dielectric body contains about 3% by weight or less CuO, more preferably, about 2% by weight or less. The copper oxides decrease the firing temperature. Due to the copper oxides being provided, a dielectric body having a high Q value can be obtained. If the content of the copper oxides exceeds about 3% by weight, the Q value may be decreased.
Preferably, the MgOxe2x80x94MgAl2O4-based ceramic powder is represented by the formula xMgO-yMgAl2O4, where x and y satisfy the relationships 10xe2x89xa6xxe2x89xa690 and 10xe2x89xa6yxe2x89xa690, respectively, and x+y=100. Consequently, it is possible to obtain a dense sintered compact by firing at low temperatures, and even when firing is performed at low temperatures, it is possible to decrease the amount of the glass power to be used, and also a dielectric body having a low dielectric constant and a high Q value in the high-frequency band can be reliably obtained. If x, which represents the weight percentage of MgO, exceeds about 90, the humidity resistance of MgO may be degraded. If x is less than about 10, the content of the expensive glass to be added may be increased in order to carry out firing at 1,000xc2x0 C. or less.
Preferably, the weight ratio of the ceramic powder to the glass powder is approximately 20:80 to 80:20, and more preferably, approximately 40:60 to approximately 60:40. Consequently, it is possible to obtain a denser dielectric body, and it is possible to prevent the Q value from decreasing by the use of the glass powder. If the ratio of the ceramic powder exceeds the above range, the density of the sintered compact may be decreased, and if the ratio of the glass powder exceeds the above range, the Q value may be decreased.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with reference to the attached drawings.